The front side of a display of a touch panel, a computer, a television set, a liquid crystal display, or the like or the front side of a decoration or the like is provided with a hard-coated film that includes a base material of a transparent plastic film and a hard coating layer that is stacked on the base material and made from an electron beam-, ultraviolet light- or heat-curable resin. The base material of the transparent plastic film is generally a transparent biaxially-oriented polyester film, and in many cases, a readily-adhesive layer is provided as an intermediate layer to improve the adhesion between the base material of the polyester film and the hard coating layer.
The hard-coated film is required to have durability to temperature, humidity and light, and to have transparency, chemical resistance, scratch resistance, anti-fouling properties, and the like. The hard-coated film, which is used for displays, decorations or the like, is also required to have excellent visibility and a certain design feature. For this purpose, a certain antireflection layer with a multilayer structure of alternately stacked high and low refractive index layers is generally formed on the upper layer of the hard coating layer in order to suppress glare, rainbow reflections or the like caused by reflected light viewed from any angle.
In applications such as displays and decorations, however, there has recently been a demand for larger screens (larger areas) and higher grades, which is particularly accompanied by an increase in the requirement level for suppression of rainbow reflections (interference fringes) especially under fluorescent lamps. Since most fluorescent lamps are of three-wavelength type for reproduction of daylight color, interference fringes can easily occur. There is also an increasing demand for cost reduction by simplifying the antireflection layer. Thus, it has been demanded that interference fringes be reduced as much as possible only by means of the hard-coated film.
It is believed that in the hard-coated film, rainbow reflections (interference fringes) can be caused by a large difference between the refractive indexes of a base material of a polyester film (for example, 1.62 for PET) and a hard coating layer (for example, 1.49 for acrylic resin).
For the purpose of prevention of interference fringes by a reduction in the refractive index difference, for example, JP H07-151902(A) discloses a method in which the refractive index of a hard coating layer is raised by adding metal oxide fine particles to the hard coating layer. The addition of metal oxide fine particles to the hard coating layer, however, leads to a decrease in the essential function of the hard coating layer, such as transparency, chemical resistance, scratch resistance, anti-fouling properties, or the like. When an antireflection layer is further formed on such a hard coating layer, the antireflection layer needs to be optimized depending on the change in the refractive index of the hard coating layer.
JP 2001-71439(A) discloses another method for suppressing the interference fringes in the hard coating layer, which focuses on local variations in the thickness of a film and includes the steps of producing a readily-adhesive film followed by calendering the film to reduce the local variations in the thickness of the film. In this method, however, the film alone is evaluated for interference fringes, and no examination is conducted regarding interference fringes based on the difference in refractive index at the interface formed by stacking the hard coating layer. This method also has an increased number of processes and thus has a problem with productivity.
JP 2002-241527(A) discloses an invention that focuses on unevenness in the thickness of the layers forming a hard-coated film and defines an interference fringe area ratio. However, the specification of this application includes no specific description on the degree of the unevenness in thickness or how to reduce the unevenness in thickness. In order to reduce the unevenness in the thickness of each layer, for example, the thickness of each layer should be strictly controlled so that there should be a problem with productivity or yield.
In addition, JP 2002-210906(A) discloses a method that focuses on the backside reflectance of a film itself and includes the steps of controlling the backside reflectance to a low level and stacking a hard coating with a specific hardness. The method disclosed in this publication, however, essentially includes the steps of forming a coating layer with a specific refractive index and a specific thickness on the side opposite to the hard coating layer of the hard-coated film and controlling the backside reflectance to 0.1% or less. Thus, even the backside must be designed when the film is designed. In addition, the backside reflectance should be controlled to be always 0.1% or less in the film manufacturing process by measuring the backside reflectance and optionally by changing the conditions in cases where the backside reflectance is out of the range, and such control of the backside reflectance complicates the process.